Uhm, why isn't there? Games having been growing in size all the time, an 8 gig bites, 1 gig byte is always possible. It is, quite obviously, easy for nintendo to make game cartridges that are that size (take a look at the 4 gig MICRO SD cards for example). Its just a case of a game needing that much data. We've already seen the need for slot 2 RAM expansions (opera browser), so the DS can certainly support that much data...
Hell even if there was a 1/2 gB game, 2 backups would take up the whole card. Thats good reason to have removable memory. Not to mention being able to switch on the fly. (ie a memory card for movies, a memory card for backups)
you will never see a 1gig game. you will also probably never see a .5gig game. at most, 256 but i doubt that. hardly any game out there has been able to use the 128 megs and even then, im sure they could use it more efficiently. MK for example was what? 30 megs? you are talking about a game that is 300x as big as MK. it wont happen. developers are lazy and know they can make money on a 50 meg game, why make a 500 meg game.
I think my only concern is how slow is slow for the transfers. and of course, as with all built in memory carts, theres the chance of games becoming bigger than that 1 gig.